Methods for collection, dark correction, and reporting of spectra from array detector spectrometers

ABSTRACT

Methods and systems for spectrometer dark correction are described which achieve more stable baselines, especially towards the edges where intensity correction magnifies any non-zero results of dark subtraction, and changes in dark current due to changes in temperature of the camera window frame are typically more pronounced. The resulting induced curvature of the baseline makes quantitation difficult in these regions. Use of the invention may provide metrics for the identification of system failure states such as loss of camera vacuum seal, drift in the temperature stabilization, and light leaks. In system aspects of the invention, a processor receives signals from a light detector in the spectrometer and executes software programs to calculate spectral responses, sum or average results, and perform other operations necessary to carry out the disclosed methods. In most preferred embodiments, the light signals received from a sample are used for Raman analysis.

REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. patent applicationSer. No. 14/728,818, filed Jun. 2, 2015, the entire content of which isincorporated herein by reference.

FIELD OF THE INVENTION

This invention relates generally to spectrometers and, in particular, tomethods for collection, dark correction and reporting from suchinstruments.

BACKGROUND OF THE INVENTION

Electronic light-recording devices such as charge-coupled display (CCD)cameras, single element arrays, as found in InGaAs cameras, and soforth, have a dark response (i.e., a signal in the absence of light)which must be corrected. Normally this involves taking an exposure cyclein the absence of light from the sample to be measured and storing it asa “dark spectrum.” Light from the sample is then passed to the camerafor an identical exposure cycle to generate an “uncorrected samplespectrum.” A “corrected” sample spectrum is then computed by subtractingthe dark spectrum from the uncorrected sample spectrum. (Other forms ofcorrection are then computed to correct for the spectral responsivity ofthe detectors, the spectral mapping of the array, interpolation, etc.,but these are separate subjects outside the scope of this disclosure.)As the time between collection of the dark and the collection of lightbecomes larger, the dark data may not match the true camera response inthe absence of light due to temperature fluctuation or other reasons.

If the dark spectrum is updated prior to each light exposure cycle, thisessentially doubles the amount of time required for a total datacollection cycle. Further, when analyzing spectra that contain both veryweak and very strong spectral components of interest, the exposure cycletime required for adequate SNR (signal-to-noise)/quantitation on thevery weak components, such as in analysis of gas mixtures by Ramanspectroscopy, can be very long—on the order of several minutes. Strongercomponents in the same mixture may be accurately quantitated in a matterof seconds.

Previous attempts to solve the dark correction problem either areinefficient in the amount of time required, or inaccurate in matchingthe true dark response at the time of light collection. Existingtechniques either collect one dark spectrum and apply it to all futurespectra in an experiment or monitoring process, or collect a new darkspectrum before each signal spectrum.

Standard Practice 1

FIG. 1 illustrates current standard practice involving a single storeddark spectrum. A collection cycle consists of N accumulations of singleexposures, each within the dynamic range of the array detector, summedor averaged to achieve a target SNR for the most difficult (typicallythe weakest) spectral feature in application. The resulting sum oraverage will henceforth be referred to as a spectrum A dark exposure isacquired with signal light blocked, thus acquiring one dark exposure at102. A second dark exposure is acquired at 104 for cosmic eventcorrection, and this process is repeated by summing or averaging thecosmic corrected exposures at 108 for N accumulations (106). Inaccordance with this disclosure, including the embodiments describedhere, “cosmic correction” should be taken to mean combining two (ormore) exposures in such a way as to eliminate pixel signal if one of theexposures show evidence of cosmic ray spikes, while averaging the pixelsfrom the both exposures if neither has a cosmic-ray-induced spike.Further, “N” is typically determined by the ratio of the strongestfeature in the spectrum to the weakest feature, such that each of the Naccumulations is sufficiently short to avoid detector saturation at thestrongest feature, and the total exposure time T over N accumulationsprovides the required SNR for the weakest component.

The resulting dark spectrum is saved at 110 and subtracted at 112 fromall subsequent signal collection spectra acquired in the same manner,but with signal light illuminating the detectors. The result is outputat 114. This approach may comprise a standard practice for sufficientlystable dark current, which can be the case for very stable dark current,typically characterized by very stable thermal environments for bothdetector and spectrograph hardware. It can also be the case forapplications with very strong signals relative to dark current. Thecycle time for data within a run is the shortest possible because oncethe single dark spectrum is acquired, signal data is being acquired atall times. Total data reporting cycle time for the method of FIG. 1 isT, as dictated by the weakest component of interest.

Standard Practice 2

FIG. 2 illustrates an alternative standard practice involvinginterleaved dark spectra. A dark spectrum is acquired at 202, followedby the acquisition of signal spectra at 204. The dark signal issubtracted from the light spectrum at 206. A new dark spectrum isacquired over N accumulations as described above in between each signalcycle of N accumulations. This allows the instrument to correct forchanges in dark current over the course of a data run. However, itdoubles the data cycle time relative to Standard Practice 1, becausehalf of the time is spent acquiring dark spectra, not signal. Thus,total data reporting cycle time is 2T.

SUMMARY OF THE INVENTION

This invention is directed to a system and method of dark currentcorrection in a spectrometer having a detector adapted to receive lightfrom a sample. The overall goal is to provide for efficient darkcorrection while keeping the total data collection cycle to a minimum.The various embodiments also enable more rapid reporting of data thanthat which would normally be dictated by accurate quantitation of theweakest signal of interest.

The invention affords better matching of dark subtraction to the truedark when light data is acquired. This results in more stable baselines,especially towards the edges where intensity correction magnifies anynon-zero results of dark subtraction, and changes in dark current due tochanges in temperature of the camera window frame are typically morepronounced. The resulting induced curvature of the baseline makesquantitation difficult in these regions.

One disclosed method allows dark data to be pre-calibrated duringextended periods of time to improve the accuracy and reduce noise, thenthese calibrations can be used at any point in the future withoutincurring an increased measurement time. Alternative methods provideadditional metrics for the identification of system failure states suchas loss of camera vacuum seal, drift in the temperature stabilization,and light leaks.

In system aspects of the invention, a processor receives signals from alight detector in the spectrometer and executes software programs tocalculate spectral responses, sum or average results, and perform otheroperations necessary to carry out the disclosed methods. In mostpreferred embodiments, the light signals received from a sample are usedfor Raman analysis.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram that illustrates a current standard practiceinvolving a single stored dark spectrum;

FIG. 2 illustrates an alternative standard practice involvinginterleaved dark spectra;

FIG. 3 illustrates an interleaved dark exposure method according to theinvention;

FIG. 4 illustrates a rolling collection method according to theinvention;

FIG. 5 illustrates a scaled dark collection method according to theinvention;

FIG. 6 illustrates an embodiment of the invention that combines theMethods depicted in FIGS. 4 and 5;

FIG. 7 illustrates a calculated full spectrum dark correction methodaccording to the invention;

FIG. 8 illustrates an embodiment of the invention that combines theMethods depicted in FIGS. 5 and 7;

FIG. 9 is a simplified flow diagram that depicts a bracketed darkmethod; and

FIG. 10 is a detailed flow diagram of a bracketed dark method.

DETAILED DESCRIPTION OF THE INVENTION Method 1—Interleaved Dark Exposure

In accordance with this embodiment of the invention, diagrammed in FIG.3, a collection cycle comprises dark exposure 302, light exposure 304,repeat dark for cosmic correction check 306, repeat light for cosmiccorrection check 308, and generate one accumulation by subtracting thecosmic-corrected dark exposure from the cosmic-corrected light exposure(310). These steps are repeated N times through decision block 312 foreach accumulation. At 314 the accumulations are summed or averaged tobuild up the target SNR for the application.

This improvement doubles the fastest possible cycle time and bettermatches the true dark for light collection periods to the stored dark ascompared to Standard Practice 2. This can be significant in applicationswhere dark current can drift significantly within a long single datacycle of N accumulations. Data reporting cycle time is 2T, equivalent toStandard Practice 2, but provides more accurate tracking of dark currentdrift than Standard Practice 2.

Method 2—Rolling Collection

In the embodiment of FIG. 4, the acquisition cycle includes performingInterleaved Dark Exposures of FIG. 3 for each accumulation, storing eachof the N accumulations in a buffer. Two dark exposures are collected andcosmically corrected at 402; two light exposures are collected andcosmically corrected at 404; with the difference 406 being stored in abuffer at 408. When desired number of exposures N has occurred (at 410),a sum or average is taken at 412 a first collect cycle spectrum resultis returned at 414, and the oldest buffer element is deleted. Steps402-408 are repeated through 416, and the result of each correctedexposure is added to the buffer as newest buffer element. Anothercollect cycle spectrum is returned which incorporates all bufferelements including the newest one.

This process is repeated as a rolling sum or average spectrum deliveryuntil a sufficient number of spectra has been achieved via block 418, inwhich case the process quits at 420. Although full reaction to a stepchange in signal level is similar to Standard practice 2, data reportingto indicate the onset of a signal change is actually faster than thecycle time of Standard Practice 1, returning a new spectrum with thetarget SNR on every accumulation, instead of every N accumulations. Thedata reporting cycle time is now 2T/N (except for the first spectrumwhich would be delivered after 2T).

Method 3—Dynamically Modeled Dark Collection

In accordance with the embodiment of FIG. 5, a dark is collected at thebeginning of an experiment at 502 using the entire data collection cycleand stored as in Standard Practice 1. This dark uses the same region ofthe camera as light collection, but with no light entering the camera,and will be referred to as the true dark (TD). Subsequently, a seconddark is collected at 504 using regions of the camera not normallyilluminated during signal collection, such as in between signal fiberimages on a 2-dimensional CCD array, or non-illuminated regions of alinear array detector. This dark is collected with light entering thecamera and will be referred to as the unilluminated dark (UD).

At 506, a relationship is developed dynamically between the TD and UD,indicated as TD=fn(UD). In some situations the functions may simply be amultiplication by constant. A light collection cycle is then started at508. Simultaneously, another UD is collected at 510 using detectorregions not illuminated by signal light. Using the previously developedrelationship between the TD and the UD, a new TD is calculated at 512using the monitored UD signal. The calculated TD is then subtracted fromthe signal exposure at 514. The result at 516 should closely match thesignal corrected by true dark during light collection. No additionalexposure time is required.

Data reporting cycle time after initial dark collection is T, which isequivalent to Standard Practice 1. Drifting dark current is nowcorrected, although not as accurately as with the Rolling Collectionapproach. If the dark current drift is reasonably consistent across thedetector array, this can provide sufficiently accurate correction. A newrelationship between the TD and the UD is developed each time theexperimental parameters (such as time of exposure or detectortemperature) change. No additional inputs to the function relating TDand UD are necessary other than the UD.

Note that in this method, the initial dark may be taken for a subset oftotal accumulations to save start-up time, but this would compromiseSNR. Also, the UD does not have to be a contiguous stripe across thecamera but can in fact be any collection of unilluminated pixels.

Method 4—Combination of Methods 2, 3

The approach of FIG. 6 essentially combines the improved Methods 2 and 3(FIGS. 4 and 5). The technique represents a rolling collection of bothsignal and dynamically modeled TD correction, reporting data on everysignal accumulation without interleaved dark collections. Blocks 602,604 and 606 are equivalent to the initialization cycle of FIG. 5, andblocks 608, 610, 612 representing the collection cycle. At 614 the darkis subtracted from the signal and the result being stored in a buffer at616. As with the process of FIG. 4, when desired number of accumulationsN has occurred (at 618), a sum or average is taken at 620, a firstcollect cycle spectrum result is returned at 622, and the oldest bufferelement is deleted at 624. Steps 608-624 are repeated through 626, andthe result of each accumulation is added to the buffer as newest bufferelement. Another collect cycle spectrum is returned which incorporatesall buffer elements including the newest one. The data reporting cycletime is T/N—Twice the speed of Rolling Collection Method 2.

Method 5—Statically Modeled Dark Correction

For cameras with a consistent dark current vs. detector temperaturecharacteristic, the complete dark spectrum response to relevantparameters, such as integration time and detector array temperature, canbe measured over the entire array and stored once in advance at selectintervals within the expected operational ranges. These parameters canthen be measured during operation, and the expected operational darksignal calculated via interpolation of the stored data. This providesthe advantage of low noise dark current subtraction, with theoperational simplicity of Standard Practice 1, although a new staticmodel would have to be developed for each instrument at the time ofmanufacture or refurbishment.

The technique is diagrammed in FIG. 7. At 702, dark response is measuredat various detector states. At 704, dark response is measured inconjunction with various detector parameters such as differenttemperatures, exposure time(s), and so forth. The responses acquired at702, 704 are stored at 706 as a specific model for that particulardetector.

The collection cycle begins at 710, wherein the signal spectrum iscollected along with the state and parametric information derived at702, 704. This allows the dark spectrum to be calculated using thestored model at 712. The calculated dark is subtracted from the signalat 714 and this is repeated N times via 716. The corrected signalexposures are summed or averaged at 718 and the result delivered at 720.Data reporting cycle time can be either T or T/N, depending on theincorporation of the rolling average method described in Method 4.

Method 6—Scale-Enhanced Statically Modeled Dark Correction

The embodiment of the invention shown in FIG. 8 represents a combinationof Methods 3 and 5. As in Method 5, a functional relationship isdeveloped at 806 between true dark (TD) and relevant operationalparameters (e.g., integration time, array temperature). In addition,another functional relationship is developed at 810 betweenunilluminated dark at 808 (where light is entering the camera but notfalling on the UD regions) and the operational parameters used in thefirst functional relationship. This will be referred to as staticallymodeled unilluminated dark (SMUD). As in Method 5, these functionalrelationships would be developed at the time of instrument manufactureor refurbishment and used for all future correction of exposures wheresignal light is illuminating the detector regions.

In this embodiment, however, the statically modeled dark correction issupplemented with a scaled dark correction factor determined from thedifference between the actual UD that is measured and the UD that ispredicted from the statically modeled unilluminated dark. This accountsfor camera instability or other operational variables not accounted forin the implementation of Method 5. This process includes statisticalmeasures to determine when the UD region differs significantly from thecalculated UD value, in turn triggering the application of an additionalscaled dark correction to supplement the statically modeled darkfunction. This approach can also provide additional benefits, such ascorrecting for interchannel smearing in shutter-free applications andhandling unexpected light leakage inside the spectrograph.

Data reporting cycle time can be either T or T/N depending on theincorporation of the rolling average method described in Method 4.

Selection of N Based on External Control System Requirements

As described above, the total number of accumulations N is typicallyrelated to the ratio of the strongest signal to the weakest signal inthe spectrum in order to avoid detector saturation on any singleaccumulation. Improved methods 2 and 4 shorten the data reporting cycleto 2T/N or T/N respectively. However, some applications may need stillfaster reporting cycles to support control system requirements. Anexample of such an application would be optimizing the efficiency of anatural gas turbine power generator based on the varying concentrationsof different hydrocarbon constituents in the gas being fed to thegenerator. In improved methods 2 and 4, the required signal exposuretime T may be divided in to a larger number of accumulations N in orderto report at a speed consistent with the control application. The numberN will be limited at some point by increasing relative significance ofdetector read noise and A/D quantization noise, as understood to thoseof skill in the art.

Component Selective Response Time

Improved methods 2 and 4 above provide more rapid indications of anonset changes in sample constituents than standard practice. However,they still nominally require time 2T or T, respectively, to fullyrespond to a step change in the sample. Methods 2 and 4 may be furthermodified such that the stronger spectral components are assigned buffersizes that are smaller than the N accumulations as described in Method2. As described above, T is dictated by the weakest component in thespectrum, whereas stronger constituents can achieve a target SNR in ashorter total exposure time. By customizing the buffer size to besmaller than N as appropriate for stronger spectrum components,detector-by-detector, the system can be made to fully respond to changesin concentration on stronger components more rapidly.

Bracketed Dark Methods

It has been discovered that additional advantages may be gained byacquiring multiple dark and multiple signal exposures, but splitting thedark collections so that half are taken before the signal exposures, andhalf are taken after the signal exposures. The total dark is thensubtracted from the signal to produce a “collection.” This has twoadvantages. First, by splitting the darks into two halves that bracketthe signal, the dark spectrum better matches the true dark contributionto the bracketed signal collection. A second advantage is that theending dark half-series of one collection is re-used as the beginninghalf-series of darks for the following collection, thus saving time. Forequivalent total number of darks to signal exposures, the data cycletime drops from 2T to 1.5T, as shown conceptually in FIG. 9.

In practice, each time interval of dark and signal integration may be asummation or average of multiple camera exposures/readouts. This isoften necessary in order to acquire a target Signal to Noise Ratio (SNR)on spectral features that might otherwise be too weak, as limited by thedynamic range of the camera, the dark current of the camera, and/orstronger signals in the spectrum.

It is also standard practice to apply “cosmic filtering” to eachexposure of dark and signal. This process splits a desired exposureinterval into two equal sub-intervals. The two sub-intervals arecompared with each other to detect any anomalous high-intensity “spikes”in one of the two spectra. These spikes occur as random low-probabilitycosmic radiation events, and discarded when detected. Any reference toan “exposure” below may also in practice be cosmically filtered dataacquired in this way.

Note that the number of dark exposures does not necessarily have tomatch the number of signal exposures if the noise inherent in the darkexposure is not a substantial contributor to the total noise inherent inthe collection. This would be the case in a scenario where the darkcurrent is much lower than the signal current. In such a scenario,additional time gains can be made by using less total dark exposuresthan signal exposures, providing signal to noise of the resultingcollection is still adequate. In this case the shorter dark exposurescan be scaled to compensate for the different total integration timerelative to the signal exposures, yielding an “effective integrationtime” of T/2 according to FIG. 9.

This bracketed dark method is applicable to any embodiment disclosedherein where multiple dark and signal collections are performed in orderto achieve a desired signal-to-noise ratio. Indeed, this modification isapplicable to the Standard Practice of FIG. 2. This improvement is alsoapplicable to the embodiments that employ a buffer. By defining a“collection” as one corrected averaged/summed signal exposure, eachcollection is placed in a buffer which can then be used to returnaveraged or summed collections as a spectrum on a rolling basis (FIFObuffer). To demonstrate, the modification is applied to Method 2, the“Rolling Collection” method shown in FIG. 4, with the modified versionbeing depicted in FIG. 10. A detailed description of FIG. 10 proceeds asfollows:

-   -   1. First a series of D cosmically corrected dark exposures are        collected. Call this set A.    -   2. A set of S cosmically corrected signal exposures (shutter        open) are then collected. D will typically be ½ S but may also        be less, down to the case where D=1.    -   3. A second set of D cosmically corrected dark exposures are        obtained.    -   4. The two sets of dark exposures (set A and set B) are then        averaged or summed. If they are summed, and 2D does not equal S,        then a scaling of the dark sum by multiplying by S/2D would        occur so that the sum is then appropriate for S exposures.    -   5. The set of signal exposures are then averaged or summed.    -   6. The average/sum of the signal exposures is then corrected by        subtracting the average/sum of the dark exposures.    -   7. The subtracted signal exposure, subsequently referred to as        one “collection” is stored in a buffer of length N.    -   8. Steps 2-7 are repeated more times until the buffer is full.        At this time the first spectrum is constructed by        averaging/summing the N buffer elements.    -   9. The oldest element in the buffer is discarded. Also the set        of darks previously referred to as set B now become set A and        the former set A is discarded.    -   10. Steps 8-9 are repeated, again filling the buffer. At this        point another spectrum can be delivered, or the buffer further        updated. The update interval (how many times the buffer is        shifted by repeating steps 8 and 9) can vary from 1 (fastest        update time, deliver a spectrum after every collection) to N        (slowest update time, no rolling average/sum occurs).        This cycle continues until no further spectra are desired.

Method Selection Based on Application

The selection of a method described herein depends on timing, accuracy,setup/computational resource priorities and application requirements.Interleaved dark collection above is the most accurate way to track darkcurrent, particularly for single row cameras with high and significantlyvarying dark current such as an InGaAs linear array camera, and also themost accurate way for a 2D array camera such as a CCD, providing bothincreased data reporting rate at target SNR, and most accuratecorrection for varying dark current. The Standard Practice of a singledark collection is a faster, providing twice the data/response rate inreturn for a less rigorous estimated tracking of dark current. TheStatistically Modeled Dark correction is the fastest overall method(including manufacturing time and end-user time), as it requires noadditional effort at time of manufacture. However, this method providesstill faster reporting of data from the viewpoint of the customer,although the customer may have to pay a charge for developing the modelas extra work is required at time of manufacture. Finally, several ofthe methods can benefit by implementation as a rolling average, ifdemanded by a process control system, without actually changing theamount of time for the system to fully respond to a step change in theprocess constituents. Finally, customization of the amount of averagingbased on process control requirements or component concentration canalso be employed.

1. A method of dark current correction in an instrument having adetector adapted to receive a light spectrum from a sample, the methodcomprising the steps of: (a) collecting a first set A of D darkexposures; (b) collecting a first set of S signal exposures; (c)collecting a second set B of D dark exposures; wherein D represents atotal exposure time of equal to or less than that of S/2; (d) averagingor summing A and B, and scaling the result for equivalent total darkexposure time to signal exposure time if necessary; (e) averaging orsumming the signal exposures; and (f) subtracting the result of (d) fromthe result of (e) to deliver a dark-corrected collection.
 2. The methodof claim 1, including the step of repeating steps (b) through (f) untila desired signal-to-noise ratio is achieved by summing or averaging themultiple dark-corrected collections from (f).
 3. The method of claim 1,including the steps of: (g) storing the result of (f) in afirst-in/first-out buffer of length N; (h) discarding set A and renamingset B as set A; (i) repeating steps (b) through (g) until the buffer isfull; and, if a spectrum result is desired; (j) averaging or summing theN buffer elements to output the spectrum result; and, if a spectrumresult is not desired; and (k) discarding the oldest element in thebuffer and returning to step (h).
 4. The method of claim 3, wherein thefrequency of step (k) is adjusted in accordance with the desirability ofoutputting a spectrum result.
 5. The method of claim 4, wherein aspectrum result is delivered after every dark-corrected collection. 6.The method of claim 4, wherein a spectrum result is not delivered afterevery dark-corrected collection.
 7. The method of claim 1, wherein thespectrometer is a Raman spectrometer, and the light spectrum from thesample includes a Raman spectrum.